Composite resins that are a typical example of dental compositions usually contain a polymerizable monomer composition and additives such as a filler, a polymerization initiator, a polymerization inhibitor, a dye, and etc. In a composite resin including such components, a filler usually has the largest weight fraction followed by a polymerizable monomer composition and these two components represent a major proportion of the weight of the composite resin. The polymerizable monomer composition serves as a binder for the filler. The properties of monomers, and the properties of cured products of the compositions are significantly influential on the properties and performance of the composite resin containing the monomer composition, and cured products thereof.
From the points of view of aspects such as the biological safety of monomers and the mechanical strength and wear resistance of cured products, the polymerizable monomer compositions frequently include radically polymerizable polyfunctional methacrylates. Typically, the polyfunctional methacrylate compositions are based on 2,2-bis[4-(3-methacryloyloxy-2-hydroxypropoxy)phenyl]propane (hereinafter, written as Bis-GMA) or 2,2,4-trimethylhexamethylenebis(2-carbamoyloxyethyl)dimethacrylate (hereinafter, written as UDMA), and contain triethylene glycol dimethacrylate (hereinafter, written as TEGDMA) to control the viscosity.
In the dental clinical practice, the use of composite resins in the restoration of tooth defects has a long history and is still expanding. However, the mechanical properties of cured composite resins are still insufficient. In particular, the poor strength obstructs the application of the resins to sites subjected to a high stress, for example, the use as molar tooth crowning materials.
In recent years, clinical experts strongly demand the expansion of the use of composite resins to such high-stress sites. Therefore, the development of composite resins having higher mechanical properties is an urgent necessity. As mentioned above, the properties of cured products of polymerizable monomer compositions used for composite resins significantly affect the properties of cured products of the composite resins containing the compositions.
Techniques have been reported in which Bis-GMA and UDMA that are widely used as main components of polymerizable monomer compositions are replaced by other monomers so as to enhance the mechanical strength of cured products of composite resins (Patent Literature 1 and Patent Literature 2).
Further, techniques aiming to improve main component monomers have been reported. For example, main component monomers are improved so as to enhance the refractive index of cured products of polymerizable monomer compositions (Patent Literature 3), and main component monomers are improved so as to enhance the degree of polymerization shrinkage between before and after the curing of polymerizable monomer compositions (Patent Literature 4).
Dental adhesive compositions, which are a typical example of dental materials, usually contain a polymerizable monomer containing no acidic groups, a polymerizable monomer containing an acidic group, a polymerization initiator, a polymerization inhibitor and other functional additives. The performance of dental adhesive compositions that is of greatest interest is to bond a prosthesis and a filling material to tooth structure. To enhance this performance, many studies report improved polymerizable monomers with an acidic group which are components considered to have a direct impact on adhesion (for example, Patent Literature 5).
A typical dental adhesive composition contains a polymerizable monomer containing no acidic groups in an amount that is equal to or greater than that of a polymerizable monomer containing an acidic group. This fact is rarely focused on in conventional techniques.
From points of view such as the biological safety of monomers and the mechanical strength and wear resistance of cured products, radically polymerizable polyfunctional (meth)acrylate compounds are frequently used as the acidic group-free polymerizable monomers. Typical examples of such polyfunctional (meth)acrylate compounds include 2,2-bis[4-(3-methacryloyloxy-2-hydroxypropoxy)phenyl]propane (hereinafter, written as Bis-GMA), 2,2,4-trimethylhexamethylenebis(2-carbamoyloxyethyl) dimethacrylate (hereinafter, written as UDMA) and triethylene glycol dimethacrylate (hereinafter, written as TEGDMA).
Dental adhesive compositions have found a wide use in dental clinical practice. A typical example is various dental adhesive materials such as adhesive cements for bonding a prosthesis to tooth structure, bonding materials, coating materials, orthodontic adhesives and mobile tooth fixing materials. Of these materials, adhesive cements are required to attain an enhancement in bond strength but have not reached a satisfactory level of this important performance. Further, such materials are known to be poor in storage stability. Dental adhesive compositions are recently being applied to new applications, for example, the fixation of mobile teeth (Patent Literature 6).
A tooth that has lost its function due to caries, an accident or the like is restored by, for example, fixing a coronal restoration material made of a metal or a ceramic such as porcelain, called an inlay or a crown, to the tooth. Such a coronal restoration material is fixed to the tooth with an adhesive called a dental cement. Of dental cements, adhesive resin cements that are frequently used are compositions which include a (meth)acrylate polymerizable monomer as a polymerizable monomer, and an inorganic or organic filler and a chemical polymerization initiator, and are cured by radical polymerization.
Some of the properties to be possessed by the above dental compositions are curability which ensures a sufficient exhibition of the performance of the composition, and excellent stability (storage stability) which allows the performance to be exhibited over a long period. Further, adhesive materials are required to have adhesion to ensure that the dental restoration material will not come off from the tooth structure after the treatment. An approach to attaining this requirement is to add an acidic group-containing polymerizable monomer as a polymerizable monomer so as to impart adhesion with respect to teeth and various coronal restorations.
To satisfy the performances described above, compositions incorporated with various polymerization initiators have been developed. Resin cements currently available in the market are divided into chemically polymerizable resins which use a redox initiator including a peroxide and a reductant such as an amine compound, and dual-cure resins which combine a redox system with a photopolymerization initiator. A chemically polymerizable resin is usually stored as separate parts composed of a peroxide-containing composition and a reductant-containing composition, and the two compositions are mixed with each other immediately before use. If the compositions contain an acidic component for ensuring adhesion with respect to tooth structure and dental restoration materials, as is the case in dental cements, dental adhesives and the like, the acidic component inhibits radical polymerization to cause a decrease in polymerization efficiency, and polymerizability is low because of the susceptibility to oxygen.
Studies report that the polymerizability of a composition containing an acidic component is improved by adding a chemical polymerization initiator which easily initiates polymerization and curing even under acidic conditions (Patent Literature 7, Patent Literature 8 and Patent Literature 9). Patent Literature 7, Patent Literature 8 and Patent Literature 9 present polymerization catalyst systems which exhibit a high polymerization curing performance even under acidic and wet conditions by virtue of containing, as reductants, a combination of an N-phenylglycine (NPG) compound that is an amine compound, and a sulfinic acid compound.
In Patent Literature 7 and Patent Literature 8, the materials are stored as a powder and a liquid, and the reductants need to be held by a specific jig or the like and be stored separately from the polymerizable monomers. That is, the manner in which the materials are stored is limited. Patent Literature 9 proposes a dental adhesive kit which allows reductants, in particular, an NPG compound that is an amine compound to be stored in a composition by the selective addition of a polymerizable monomer which is not gelled even in the presence of the NPG compound. The composition proposed in Patent Literature 9, however, is designed to withstand long storage in a cold and dark place (about 4° C.) and is still to be improved in terms of storage stability at higher temperatures such as room temperature (about 25° C.).
Patent Literature 10 describes the utility, in a photopolymerizable liquid adhesive composition, of a methacrylate compound with a specific structure represented by the general formula (1) described later wherein the structure represented by the general formula (2c) described later includes RB in the general formula (3c) described later.
Patent Literature 2 describes the use, in a high-strength dental composition, of a methacrylate compound with a specific structure represented by the general formula (1) described later wherein the structure represented by the general formula (2c) described later includes RC in the general formula (3c) described later.
Further, Patent Literature 11 describes the utility, in a dental filling material, of a methacrylate compound with a specific structure represented by the general formula (1) described later wherein the structure represented by the general formula (2c) described later includes any of RD and RE in the general formula (3c) described later.
However, no studies exist which report that a methacrylate compound with a specific structure represented by the general formula (1) wherein the structure represented by the general formula (2c) includes any of the alicyclic structures RB and RC and the aromatic ring structures RD and RE in the general formula (3c) is effective for enhancing properties, in particular, storage stability of a dental adhesive curable composition.
Sufficient curability of a curable composition that contains an acidic component which imparts adhesion to the composition is largely attributed to reductants such as an amine compound and a sulfinic acid compound. It is therefore necessary to ensure that the amine compound and the sulfinic acid compound exist stably in the adhesive curable composition so that the composition will attain storage stability.
A known approach to controlling the deactivation of an amine compound is to remove acidic components in the paste and to disperse the compound as particles in the composition. On the other hand, a sulfinic acid compound is known to be deactivated mainly by undergoing addition reaction with a double bond moiety of a polymerizable monomer (commonly known as “Michael addition reaction”). Non Patent Literature 1 mentions that the reaction rate of Michael addition reaction varies significantly depending on the types of (meth)acrylates, indicating that the selection of an appropriate type of a polymerizable monomer is an effective approach to controlling the deactivation of a sulfinic acid compound.
Another example of the use of dental adhesive compositions is mobile tooth fixing materials.
Gingival retraction associated with age or a progress of periodontal disease makes it difficult to support the teeth sufficiently, resulting in loosening and exfoliation of the teeth. Such loose teeth are called mobile teeth. Mobile teeth are treated by fixing the mobile teeth to healthy teeth to restrain the loose teeth and eliminating the cause of the gingival recession while the loose teeth are being fixed. An example material used to fix a mobile tooth to a healthy tooth is a mobile tooth fixing material. The mobile tooth fixing material is required to be resistant to breakage or exfoliation when it is distorted by an action such as mastication of food or brushing or cleaning of teeth during the fixing period. To meet this material need, the mobile tooth fixing material is required to have excellent flexibility so that a cured product of the material will follow a distortion and is also required to have excellent strength and toughness so that the cured product will not be destroyed or deformed. Further, good adhesion is also required because the fixation to a healthy tooth involves bonding to tooth structure and this bonding needs to persist for several months depending on the length of the fixing period.
One of the mobile tooth fixing materials currently used is Super-Bond (manufactured by Sun Medical Co., Ltd.). This product has been used widely as a mobile tooth fixing material because of its excellent adhesion to tooth structure and appropriate strength, flexibility and toughness against external stress. Super-Bond is a chemically polymerizable product composed of three components: liquid, powder and catalyst V, and thus the use thereof involves complicated handling and a waiting time for curing. Mobile tooth fixing materials having flexibility have been reported (Patent Literature 12 and Patent Literature 13), but it cannot be said that they have sufficient performances in view of the fact that they have a poor balance among flexural strength, flexibility and toughness and are more prone to breakage or exfoliation than Super-Bond.
From the foregoing, requirements for mobile tooth fixing materials are that cured products thereof exhibit excellent strength, flexibility and toughness and have good adhesion with respect to tooth structure, and that the material is a one-part composition so that it can be used with simple handling. Further, photopolymerization curability makes it possible for the user to cure the material at the desired time, realizing a marked enhancement in handleability.
In general, from points of view such as the biological safety of monomers and the mechanical strength and wear resistance of cured products, a polymerizable monomer composition used in a dental adhesive composition frequently includes a radically polymerizable polyfunctional methacrylate. Typically, the polyfunctional methacrylate compositions are based on 2,2-bis[4-(3-methacryloyloxy-2-hydroxypropoxy)phenyl]propane (hereinafter, written as Bis-GMA) or 2,2,4-trimethylhexamethylenebis(2-carbamoyloxyethyl) dimethacrylate (hereinafter, written as UDMA). Attempts have been reported in which the mechanical strength of cured products of such a dental composition is enhanced by using monomers alternative to Bis-GMA and UDMA (Patent Literature 14 and Patent Literature 2). However, an enhancement in mechanical strength tends to be accompanied by decreases in flexibility and toughness, and consequently the balance among flexural strength, flexibility and toughness is unsatisfactory.
Patent Literature 15 discloses a photopolymerizable orthodontic resin composition. This resin composition is suited for the fabrication of splints or bite plates used in the dental treatment for troubles such as jaw movement dysfunction (temporomandibular arthrosis), bruxism and occlusal abnormality.
A cured product of this resin composition contains a crosslinked polyurethane powder in order to attain appropriate elasticity, but has no adhesion with respect to tooth structure and thus cannot be used as a mobile tooth fixing material which requires adhesion with tooth structure.
As described above, there has been a demand for a composition which has excellent strength, flexibility and toughness, exhibits good adhesion with respect to tooth structure, and is a one-part formulation so that it can be used with simple handling.
Dental curable compositions are used as dental adhesives, coating materials, filling or sealing materials and the like in dental clinical practice.
Dental curable compositions are used to fill chips or cavities in teeth or to fill caries, or are used as adhesives to fix a metallic or ceramic coronal restoration material called an inlay or a crown to a tooth. Such a composition generally includes a polymerizable monomer, a radical polymerization initiator and a filling material such as a filler. When adhesion with respect to adherends is required, the composition frequently contains an acidic component. Such dental curable compositions are required to have adhesion to prevent the exfoliation of fillings, and to have sealability to prevent the entry of contaminants from the bond interface. However, an acidic component present in the dental curable composition generally serves as a factor inhibiting radical polymerization. Further, water and oxygen which are abundant in the mouth are generally inhibitory to radical polymerization. Dental curable compositions are required to exhibit the performances described hereinabove in the presence of these polymerization inhibitory factors. Compositions incorporated with various polymerization initiators have been developed to attain such performances.
Patent Literature 7 and Patent Literature 8 show that radical polymerization is allowed to take place without problem even under wet and acidic conditions by the use of an aromatic amine compound having a nonaromatic carbonyl group, and an organic sulfinic acid compound. Further, Patent Literature 3 presents that the use of an alkali metal or an alkaline earth metal makes it possible to attain enhanced storage stability while an aromatic amine compound having a nonaromatic carbonyl group, and an organic sulfinic acid compound are present in a paste.
Although Patent Literature 16 proposes a technique which enhances storage stability in a paste state, the technique of Patent Literature 16 assumes that the paste is stored under refrigeration. Radical polymerizability persists even during room-temperature storage as long as the storage is about 2 years long, but the polymerization rate is decreased significantly. It can be said that a need for further improvement is arising in light of the recent dental market environment which more often requires that dental curable compositions have thermal stability and can be stored at room temperature.
From literature such as Non Patent Literature 1, it is known that a dental curable composition containing an organic sulfinic acid compound is generally degraded to a significant extent during room-temperature storage by the nucleophilic addition of the organic sulfinic acid compound to (meth)acrylate frequently used in dental curable compositions. Non Patent Literature 4 reports that an organic sulfinic acid compound having an electron withdrawing group has a low rate of nucleophilic addition reaction but the polymerization promoting effect of the organic sulfinic acid compound is low at the same time. It is easily inferred that a dental curable composition containing such a compound is likely to suffer a curing failure and eventually a bonding failure when applied to a wet environment such as at an interface with tooth structure.
Regarding dental curable compositions containing an organic sulfinic acid compound and a polymerizable monomer, none of the literatures describes the stability of bond strength over a long period at room temperature or the stability of bond strength in an accelerated test at a higher temperature that ensures the stability of bond strength at room temperature.